The episode "The Reluctant Stowaway" identifies Dr. Zachary Smith as an intergalactic doctor of environmental psychology. He was the United States Space Corps' staff psychologist and environmental expert prior to his unexpected departure from Earth. He was an agent for an enemy foreign government.

Prior to the launch of the Jupiter 2 from Earth, Dr. Smith reprogrammed the Robot to destroy the ship eight hours after departure. While making last minute adjustments to ensure a successful sabotage, Dr. Smith subdued a guard on duty aboard the spacecraft, possibly killing him. He was later trapped aboard the Jupiter 2 while attempting to reactivate the Robot which had been shut down by a technician. The crew were protected from the effects of lift-off in their state of suspended animation, but Dr. Smith was forced to endure it fully conscious. The ship is thrown off course by Dr. Smith's additional weight and becomes hopelessly lost. It was later revealed in the series that this course deviation prevented the destruction of the Jupiter 2 in a violent meteor shower soon after lift-off.

Dr. Smith spent his earliest years in lower Manhattan, New York City, until his parents were killed in a boating accident. The young Smith then went to live with his Great Aunt Maude and Great Uncle Thaddeus in Marietta, Georgia. Aunt Maude was the matriarch of the Smith family, and she raised Zachary with a stern hand. In his own words, the doctor admitted that some times he had 'a tendency to be lazy,' but the determined lady pressured him into obtaining reasonable grades at school. Through her considerable social connections, she managed to obtain Marshall scholarships for the entrance of both Zachary and his cousin Jeremiah to Oxford University. At Oxford, Smith worked on his degree in psychology, and became the Grand Master of the Oxford Chess Society for three years in a row. He later entered Harvard University where he earned his doctorate. After graduation, he joined the United States Air Force.

Over the next 16 years, Smith had an exceptional career as a military psychologist, and was stationed at a number of U.S. military bases in the United States and Europe. He was promoted to Colonel in 1995, and then transferred to the recently formed United States Space Corps. The USSC sent him to its Houston, Texas, training center, where he went through an extensive paramedical retraining program. The training combined his expertise as a psychologist with a new branch of medicine dealing with the mental stress and physical pressures that would be put on future space colonists. Only about two dozen people in the world specialized in this field, known as Environmental Space Psychology.

Dr. Smith was involved in the artificial intelligence programming of the Jupiter 2's Series M-3, Model B-9 robot. He reviewed the psychological fitness of potential pilots for colonization missions, as well as volunteers for the missions, including the Robinson family. Smith performed the final stress-analysis examinations of the Robinsons before they left Earth.

The above information was taken from the standard USSC personnel file, but there was another side to Zachary Smith, which Alpha Control officials pieced together after they began to suspect that Smith's disappearance at the time of the Jupiter 2's ill-fated launch was more than a coincidence.

Smith's Aunt Maude's husband, the late Thaddeus Smith, was an eccentric, superstitious and bizarre person who had a bad influence on young Zachary and his cousin Jeremiah. It was discovered that Smith's high grades were largely due to cheating and bribery, and that his entrance to Oxford was due to a bribe paid by his prosperous Aunt. Zachary and Jeremiah hadn't wanted to attend college in England, but Aunt Maude, an Oxford graduate herself, convinced them by promising that whoever graduated first would inherit her fortune. Within six weeks of entering Oxford, Jeremiah was expelled for gambling and returned to Georgia. Shortly thereafter, Aunt Maude died mysteriously. Jeremiah disappeared, amid rumors about a UFO abduction.

Under the circumstances, the terms of Aunt Maude's will were revised so that the last surviving Smith cousin would receive her fortune. Zachary, without the comfortable income he had become accustomed to, dropped out of Oxford for a while, and traveled around Europe as a common drifter. During this period, he was apparently contacted by an intelligence agent of a country that considered itself an enemy of the United States and was offered a substantial sum of money to act as a future plant within the United States Air Force. Smith returned to the United States with a degree from Oxford (now known to be forged). His intelligence contacts saw to it that he was admitted to Harvard.

Smith later used his position as a military psychologist to learn classified information from his patients using hypnosis and other means. His cover was obviously a good one, for while Smith never made many friends with his caustic personality, he did develop an excellent reputation as a doctor, and no one ever suspected his true purpose within the military. During his military career, Smith's tastes became very expensive, and he pressured his intelligence contacts to get him a position that would gain him greater income. They installed him as an operative within the USSC, along with several other operatives already there, collectively known by the code name Aeolus Umbra. Note: Aeolus was the keeper of the winds in ancient Greek mythology; umbra is the Latin word for shadow or shade, and, by extension, ghost or spirit.

The destruction of the Jupiter 1 in 1993, which was officially attributed to a fuel system malfunction, was probably due to sabotage by these agents. Although Smith was not involved in that disaster, he undoubtedly had help from other agents within the Space Corps with his attempt to destroy the second colonization ship.

After the launch of the Jupiter 2 on October 16, 1997, Smith's behavior became increasingly bizarre and childish, according to logs that John Robinson left on interstellar fuel barge F-12, which were later retrieved by Alpha Control. Within a year, the enormous stress of his space voyage, which has included contact with numerous hostile aliens, had transformed Smith into a mere shell of his former self.

The final paragraph of the Alpha Control profile of Smith reads as follows:

Now that the investigation into Col. Smith's background has been concluded… the decision is up to the President as to whether Smith should be tried in absentia for murder, attempted murder, espionage, etc. Considering the negative publicity such a trial would receive, the current mental condition of the man, and the simple fact that Smith will probably never return to Earth to have whatever sentence he received carried out, we can only recommend that the man not be brought to trial unless he returns to his home planet.

The radical change in Dr. Smith's character is attributed to the magnetic fields generated by the ship's propulsion device, which can, and will, produce permanent brain damage in any life forms not protected by the freezing tube during full-powered liftoff. As could be predicted, the use of 100% of engine power when leaving Earth's atmosphere affected the unshielded doctor's mind. Within two or three weeks the behavior of the colonel grew more and more bizarre, and within a year, his exposure to the magnetic fields that propelled the interstellar ship would leave the man a shell of his former self. The Robinsons went through vigorous testing when they were chosen to become the first pioneer space family; Smith, however, received none of this testing. Like astronauts of today, the Robinsons would have had to pass many psychological tests to ensure their fitness to serve aboard a long-duration mission. Smith had none of that testing, and a good case could be made that all the stress of the mission on an untrained person, not to mention the numerous dangers with which Smith came into contact on Priplanus, finally caused him to begin losing his mind little by little. Smith was also a xenophobe who was constantly being forced into contact with aliens. This alone could also have caused a personality change.

Little is known of Dr. Smith's background. We know he had a great-aunt Maude and cousin Jeremiah Beauregard Smith ("Curse of Cousin Smith"), a great-uncle Thaddeus, possibly an uncle Thaddeus, an aunt Matilda (all three from "Ghost in Space").

In "Blast Off into Space," Dr. Smith claimed that his great-uncle Thaddeus discovered the Comstock Lode all by himself (the Comstock Lode was discovered in 1859). He also mentioned that his great-great-grandfather was a forty-niner.

As a youth, Smith spent many summers on his Uncle Thaddeus's farm where he won first prize at the County Fair but not for the tomatoes (and the Robot seems to know this in "The Golden Man"). Aunt Maude died under mysterious circumstances, Smith was at her bedside; Jeremiah gave her medicine shortly before she died but a coroner's inquest found him not guilty of foul play.

Maude left a will that gave the last remaining Smith her fortune. The last time the cousins (there were ten of them at that time) tried to settle this peacefully, Jeremiah tells John (and he could be lying) it looked like a bloodbath that made the St. Valentine's Day Massacre seem like a picnic.

On his mother's side the name is Ruthven—at least partly Scottish. One of the Smiths was a Ruthven who, in 1497, bore witness against Hamish Rhu-Glamis, ninth Laird of Glamis. Hammish was subsequently executed for high treason against his liege lord and master James Steward, king of Scotland (from "The Astral Traveler").

In "The Galaxy Gift," Smith claims to Penny that he has an aunt who lives on Knob Hill, not far from San Francisco's China Town. This might be an aunt on his mother's side, a surviving Smith not known to anyone else, or perhaps a figurative aunt (close friend to the family?) or someone else entirely—a great aunt maybe.

Smith also has a gambling addiction. A look-alike for Dr. Smith, the space outlaw and desperado Zeno of Casseopia, once made an illegal visit to the planet Earth. Smith wears a large ring on his left hand. He also seemed to have his grandfather's timepiece on him or with him since "The Reluctant Stowaway" all the way to "The Space Primevals" where a caveman alien guard snatched it from him.

Dr. Smith always loved food and lazing about. Smith also recalls walking hand in hand through nature's wonderland on nice days on Earth but he also claims to abhor nature ("The Great Vegetable Rebellion)—it always makes him sneeze (and he does so in "Deadliest of the Species" and "The Dream Monster" cliffhanger into "The Golden Man" where Penny has flowers near his nose).

Smith claims to have been a medical doctor, a psychologist, a psychiatrist, an expert gourmet as well as a doctor of environmental space psychology. He claims to have known a famous British female cook.

Smith gave teachers apples when he was in grade school, snitched on fellow school mates (such as Johnny Sorenson), played the ukulele (which he used to hide stolen test papers in when in high school and college), sold stolen test answers for 100 dollars, once wore a sportsman shirt with a C on it, and a baseball cap.

Smith may have taught NY Dodger baseball player Sandy Kofax the dipsy doodle throw (which he professed he did to Will and Penny by the time of the cliffhanger before "Rocket to Earth"). Smith seemed to disdain the mention of baseball in "Wild Adventure" ("Oh yes, THAT."). He also professes to have been taught a thing or two about pitching from Satchel Page, a Cleveland Indians baseball player who had struggle up in the league because he was black (this was just after Jackie Robinson). Smith also mentions Joe DiMaggio, Ted Williams, and Babe Ruth. All this despite Smith claiming to dislike sports in "The Deadly Games of Gamma 6"—"they seem so healthy."

Smith hit and may have killed a guard on the Jupiter 2 in "The Reluctant Stowaway" (in the original script it was a stun needle on his ring or some kind of poison), was fairly selfish and evil minded in most of "The Reluctant Stowaway," "The Derelict" (where he kills or injures a few of the bubble aliens—which he feels sorry for (somewhat) in "Prisoners of Space"), and "Island in the Sky" (where is at his meanest—forcing Don and Maureen to abandon John on the planet, using the Robot to threaten to crush their skulls, and sabotaging not only John's rockets but the Jupiter's as well). He did lament having to do away with Will, a somewhat fit companion in all this nightmare.

After the crash landing, he plots with the Robot, who he has given orders to check on his safety every hour on the hour, to kill all the others with the exception of Major West. When they learn what they needed to know from West, he, too was to have been eliminated.

Smith regretted only having to kill Will. Smith was a bit more open minded in "There Were Giants in the Earth" and "The Hungry Sea" even though still hoping the Robinsons would be killed as they ventured across the planet by lightning, earthquakes, monsters, deep freezes, and excessive heat. Yet, he warns them, not once but twice, feeling perhaps that he needed human companionship—the Robot was still pretty mechanical and dullard-like.

Smith was selfish in "Welcome Stranger," continued to spy on Penny in "My Friend, Mr. Nobody," offered Will up to the "Invaders from the Fifth Dimension" in what must be one of his most evil acts, and continued to plot against the entire group in "The Oasis."

In "The Oasis" it is apparent that John, Maureen, and the three kids have begun to feel something like respect and even love for Dr. Smith and it could also be that the fruit Smith ate, affected his mind for a bit. He certainly plots the end of the others when he thought he was going to die.

He was not too nice to Will in "The Sky Pirate" and "The Questing Beast" offering Will up as a hostage in both and not caring if the Robot killed Tucker or accidentally hurt Will in "The Sky Pirate."

He did seem to care that Penny would be hurt in "My Friend, Mr. Nobody." He spied on Judy and Penny in "The Magic Mirror" since his greed made him want the full length alien mirror the two girls were propping up for use. Smith once called Penny Penelope when he thought she was using this mirror to trick him.

He once told Penny her garden would whither and die if the Robinsons left him on Priplanus ("Attack of the Monster Plants) and in the same episode used Judy's vanishing as blackmail on the family so that Don would take him back to Earth.

He also spied on Judy and Don as well as John and Maureen in "The Keeper." Getting back to Earth, riches, power, food, and self were Smith's chief obsessions. He plotted many a time to steal some alien or visitor's spaceship ("The Keeper," "The Sky Pirate," "The Questing Beast," "Rocket to Earth," and many, many more).

Smith may have not admitted it often ("your family my boy, not mine" in cliffhanger of "Wild Adventure" into "The Ghost Planet") but he may have considered himself one of the family by the time of "The Girl from the Green Dimension." Smith was always cautious of the aliens and often misjudged the good ones—such as in "The Sky Is Falling" and "Wish upon a Star" and was good at getting the others or some of the others to his way of thinking ("The Sky Is Falling" where he has Judy and Penny thinking he is right and has Don so on edge that Don nearly curses at him and tells him to shove off of it).

Smith also misjudged the bad aliens—as in "The Space Trader." On occasion, his frantic warnings proved to be correct ("The Raft," "Wreck of the Robot," and others) but more often than not, he sounded the alarm for no good reason (as in "One of Our Dogs Is Missing).

Smith used the Robot in the early episodes as his companion and protector but by "War of the Robots," he was treating the Robot quite badly, even admitting this at the end and making up for it by giving the Robot a rubdown with oil for two weeks.

By "Ghost in Space" Smith's mind may have completely been changed into a less evil but still silly and selfish man. The takeoff in "The Reluctant Stowaway" may have affected his personality and mind since he was not protected as the Robinsons were from the acceleration (but that doesn't explain future takeoffs).

Smith was also traumatized in "The Magic Mirror" which he refused to believe wasn't his own nightmare world. Smith left Will in the bog in "Ghost in Space," left Will green in "The Girl from the Green Dimension," left Penny alone with Ohan in "All that Glitters," and did various other inane things, screaming at any remote danger or appearance of anything remotely alien.

Smith sometimes was kicked out of camp ("Wish upon a Star," "Attack of the Monster Plants," "The Dream Monster," and "The Mechanical Men"). Due to being ostracized with the silent treatment, he also removed himself in "Mutiny in Space." He removed himself before this at another time in "The Dream Monster" where no one protested his leaving.

There was some question as to whether or not he would leave the planet with them in "Attack of the Monster Plants" but most of this seems to be a ploy to get Smith to behave or a joke from Don—which Judy thinks is cruel. Smith is also left to wonder in "Cave of the Wizards" if the Robinsons would have returned for him if they had been aloft when they spotted him fleeing the alien cave and its influence over him. John seemed to think he would have to remain out in the wilderness thinking about this (another exile?).

Don wanted to leave him on the Xenian space probe and voiced at least three times in "Kidnapped in Space." Smith realized they were always threatening to leave him somewhere in "Time Merchant" ("They were always threatening to leave me somewhere.") In the same episode, Smith's inherent goodness came through and he couldn't allow Judy, Penny, and his dear William to perish, rushing back onto the Jupiter 2 to save their futures, even though it meant his being lost in space again.

Smith threw in with Sesmar in "The Dream Monster," but worked with Don to escape and get the Robinsons' emotions back.

Smith lost his mind when the Jupiter landed on Earth in the year 1947 through a time warp in "Visit to a Hostile Planet," capturing Will and Robot, trying to force the others to stay on Earth in this time to be the masters of it.

Smith may or may not have had medical experience (he seems to know what he is doing in "The Reluctant Stowaway," but Don seems to chide him for not having it in "Kidnapped in Space," but perhaps Don meant mechanical medical history and experience as the patient was robotic).

Smith accidentally causes the ship to crash in "Island in the Sky," "Forbidden World," and "Hunter's Moon" (as Robot says, "A familiar behavior pattern is noted."). He also caused it to go off course some time before "Time Merchant" and in "Wild Adventure" made it lose the auxiliary fuel supply, changed its course toward Earth several times, caused it shake uncontrollably and almost caused a fire. In the same episode, he set it toward the Earth's sun.

He also, in the cliffhanger, places the ship in a deadly radiation belt which endangers it for its ride, also thanks to him to the dangerous "Ghost Planet." Smith was sort of taken over again in "Space Destructors" when he wants to rule the world using an alien machine that makes cyborgs. It seemed to bring out his more evil side. John almost exiled Smith for good after Will was almost turned into a cyborg for all time. Smith's true apology to John made John give him another chance.

Smith's humanity came out in "The Questing Beast" when he regretted every lie he ever told when Will wouldn't believe the truth he told now. Smith also wanted to save Will's youth and innocence in that episode, one of the first times he acted totally unselfishly and he admitted to Will that rushing to grow up isn't all that worth it.

The other time Smith was somewhat evil was when he was being used by the "Space Creature." Smith was turned into a hippie ("The Promised Planet"), a celery stalk ("The Great Vegetable Rebellion"), and a super strong Samson type ("Collision of Planets").

Smith also tried to take over the "A Day at the Zoo" and keep Judy, Don, Will, and Penny as well as the Robot (whom Penny talked into helping them) as specimens in the cages. Don used his laser to threaten Smith in Zoo and "The Flaming Planet," something Smith once almost did to Don in "The Reluctant Stowaway."

Smith abhorred guns as early back as "Welcome Stranger" but took one anyway. By the time the show ended he wouldn't really carry one or even hold one.

Smith tricked Judy into becoming a "Space Beauty" in Farnum's beauty contest unaware it would have dire consequences for her and sold the Robot to "The Space Trader" and the Junkman of "Junkyard in Space" for food both times.

He lied to Maureen in "Return from Outer Space" about the omelet, wanted to see the future in "The Girl from the Green Dimension," conned Will and Penny and Don and almost every member of the Jupiter 2 at one time or another, and made countless errors which endangered everyone—not on purpose most of the time ("Mutiny in Space," "Trip Through the Robot," "The Mechanical Men").

Smith's double Daddy Zack (who was made out of all the inherent goodness of Smith) sang "I've Been Working on the Railroad," a song Smith sang in "The Phantom Family."

Although the most cowardly member of the group, Smith's bravery can be seen in "The Space Primevals" (where he saves Don and bonds with the Major, calling him Don for once), "The Flaming Planet" (where he does save the Robot's sensory tapes), "Time Merchant" and "The Raft" (where he does pull Will away from the dangerous plants).

At first, wildly evil ("The Reluctant Stowaway," "The Derelict," "Island in the Sky," "There Were Giants in the Earth," "The Hungry Sea") he began to mellow by "The Hungry Sea," and although slipping back to being malicious self centered ("Attack of the Monster Plants") and greedy ("My Friend, Mr. Nobody") as well as truly evil minded ("Invaders from the Fifth Dimension"), Smith really didn't think until it was too late (he cared about the fate that befell poor Judy in "A Visit to Hades," worried about what he did to Will in "Ghost in Space," "Space Destructors," and worried about the Robinsons being hypnotized into leaving Penny and Will with monster aliens on "The Promised Planet"), and generally was sorry for the harm he would cause.

Smith lied about changing his ways in "Island in the Sky" (Saint Zachary). After what he thought was "A Visit to Hades" he promised to become St. Pious the third. He also made similar promises in "All that Glitters" ("I'm going to finally be the man that you all want me to be.") and "The Galaxy Gift" (but by this time Maureen didn't count on it). He also wouldn't promise John he would try to change in "Space Destructors" (the worst reversal of his behavior pattern to date—where he wants to rule the universe with cyborgs). Both men knew he couldn't change.

Smith also prayed that he would be a hard worker and a noble man in honor of Will, whom he thought was dead in "Ghost in Space." By "Two Weeks in Space" and "Castles in Space" Smith did seem to be changed somewhat, more an ally than a reluctant castaway, even though he would still dupe anyone anytime to get back to Earth or to get riches.

He did try to save Will from Chavo in "Castles in Space," was unaware of just how dangerous Judy winning the beauty contest would be in "Space Beauty," went with Robot to find Will and the real Professor in "The Anti-Matter Man," helped Will overcome the duplicates in "Target Earth" (although here he told Earth once he left the Jupiter 2 they could blow it up. Will and Robot were still on board), and cared for Don in "Fugitives in Space," not wanting to abandon the hurt Major.

By "Junkyard in Space," he was not the same man, not entirely different (he did sell the Robot to the Junkman). He told Don in "The Space Primevals," he was trying to find a way out of the cave for both of them by enticing a caveman with is grandfather's time piece. This appeared to be true.

Smith saved Don's life in the same episode. Despite haggling both Don and John and harboring some ill thought at them for their being lost—he didn't really blame them ("The Promised Planet") and looked toward them for their expertise (asking John about plans for escape in Kidnapped as well as other episodes; asking Don about hope in the crash landing in "Hunter's Moon"). He appeared to trust in their abilities to lead...most of the time. Other traits include Smith not being very good at math ("Visit to a Hostile Planet").